Thursday, May 9, 2013

Life after death: When does your phenotype expire?

Throughout an organisms’ life, the expression of genes,
regulated by the biotic and abiotic environment, gives rise to traits that
determine how fast it can run or how tall it can grow. Many traits also affect
species interactions; for example, are you fast enough to outrun predators? Do
you look tasty to herbivores? Most traits (e.g., running speed) cease to be
important once an organism dies, but some traits linger and have “afterlife”
effects on the environment. A prominent example of afterlife effects can be
found in decomposing plant material, which is a crucial part of nutrient
cycling. Microbes and fungi are critical to many stages of nutrient cycling,
such as the mineralization of organic matter and the nitrification of NH4+,
which plants cannot use, to NO3-, which is usable by plants. However, microbes
and fungi can be “picky eaters” in a sense, as they prefer substrates with
labile simple sugars instead of defensive molecules such as lignin. Simple
sugars have carbon and nitrogen supplies that are easily accessible, while
larger, more complex molecules require degradation by energetically-costly
enzymes. Therefore, genetic and environmental influences on the chemical
composition of plant material can persist after a plant sheds its leaves and
affect how quickly its nutrients are cycled.

Afterlife effects aren't a new concept; exposure
to herbivores and ozone has been shown to indirectly affect decomposition by
altering leaf chemistry. However, we recently documented a new type of
afterlife effect by showing that genotypic variation in a focal plant’s
neighbors could affect the chemical composition of the focal plants litter.
Although we don’t have the mechanism completely nailed down, it appears that
focal-plant biomass allocation (putting carbon into roots vs. rhizomes vs.
stems, and so on) is affected by neighbor-plant genotypic variation, and that
shifts in focal-plant biomass allocation are correlated with focal-plant litter
quality (specifically, lignin:N). This type of afterlife effect can also be
considered an indirect genetic effect (technically, aninterspecific indirect
genetic effect because the neighboring plants belonged to different species),
through which the expression of genes in one individual affects the phenotype
of a different, heterospecific individual.

Solidago altissima, one of the study's focal species, along the TN-NC border.

We (Jen Schweitzer, Joe Bailey, and me) were curious
whether genetically­-based afterlife effects were unique. Do they
have consequences that, for example, ozone-driven afterlife effects would not?
Ultimately, we started to think about ecosystem processes (productivity,
nutrient cycling, among many others) and the basic drivers of these processes.
We argue that ecosystem processes are the “gene-less products of genetic
interactions”, meaning that the plant, animal, and microbial traits that
interact to create ecosystem processes all have a genetic basis, although the
expression of that genetic basis may change depending on how an organism’s
genes interact with the biotic and abiotic environment. So, you may ask – “What
does this mean?” We’d argue that the “ecosystem processes are gene-less
products” perspective allows us to put ecosystems in an evolutionary framework.
We can then ask questions like: How might nitrogen cycling or plant
productivity change as natural selection acts on the genes that are the most
basic drivers of these processes?